Expansive-Shrinkage Characteristics And Mechanical Damage Characteristics Of Expansive Clay Contaminated By Cu²⁺

The process of industrialization in China has brought heavy metal pollution to the environment.At present,the research of heavy metal contaminated soil is an important field of geotechnical engineering.In this paper,the expansion and contraction characteristics,mechanical properties and micro morphology of Ningming expansive clay contaminated by heavy metal Cu²⁺were studied in detail.The main research contents are as follows.?1?The characteristics of Ningming expansive clay were briefly described.Referring to the preparation method of contaminated clay,CuSO₄.5H₂O crystal was used as the pollutant,and the samples were prepared by the method of blending and soaking with salt.The initial water content was 23%,and the concentration of the contaminated solution were 0g/L,2.5g/L,5g/L and 10g/L.?2?The expansion and contraction time history curve of Ningming expansive soil can be divided into three stages.The expansion time history curve can be accurately described by the combination of does response model and power function.For the contraction time history curve,the Does Response Model can describe the whole process of deformation well.?3?The expansion and shrinkage of expansive clay before and after contamination are compared and analyzed.The results show that the free expansion rate,expansion water content,no-load expansion rate and vertical contraction rate are significantly increased after Cu²⁺contamination.The reason is that the erosion of Cu²⁺destroys the cementation of oxides,which leads to the obvious increase of expansion and contraction.The destruction of cementation reduces the particle size of clay.Pores develop and it lead to increase of the specific surface area of particles and water absorption.Thus,expansion water content increases.?4?The microcosmic characterization test show that Cu²⁺can reduce the thickness of double electric layer,zeta potential,the distance between crystal layers of montmorillonite minerals and repulsive potential energy between particles,so the liquid plastic limit decreases with the increase of the concentration of polluted solution.However,due to the corrosion and decomposition of the contamination on the oxides,the expansion and shrinkage of expansive clay increase with the increase of Cu²⁺concentration.?5?The triaxial test results show that with the increase of Cu²⁺concentration,the damage variable increases and the shear strength decreases.The stress-strain relationship of contaminated clay is still strain hardening type,and the increase of Cu²⁺concentration has no obvious effect on the hardening trend of expansive clay.It depends on the combined effect of thickness of bound water film and the dissolution degree of free oxide.?6?The microstructure of Ningming expansive clay is arranged in the form of face-to-face superposition.With the increase of Cu²⁺concentration,the arrangement structure is more obvious.It is the result of the serious destruction of cementation.It also shows that although heavy metal pollution makes the bound water film thin and the lubrication weakened,due to the destruction of clay cementation and the development of pores,the mechanical strength of contaminated clay still shows a trend of attenuation.